Where are my credits?
On your account page, next to your plan. Credits never expire.
I paid and nothing arrived
Credits land when the payment clears, a moment after the card screen finishes.
Cancelling a subscription
From your account page, which opens the billing portal. Future charges stop; you keep what you've paid for until the period ends.
Do I have to pay?
No. Sharing a link yourself is free and unlimited. Paying is only for autopilot.